NASA managers today cleared the shuttle Atlantis for launch Friday despite a suspect electrical generator, deciding the risk of an in-flight fuel cell shutdown that could prompt a shortened mission was not a credible threat to the thrice-delayed space station assembly flight.

If all goes well, engineers will begin loading a half-million gallons of rocket fuel into the shuttle's external tank around 1:45 a.m. Friday for a launch attempt at 11:40:37 a.m. The forecast calls for a 70 percent chance of good weather, but if problems prevent liftoff, NASA can make a final try Saturday at 11:14:55 a.m., the newly declared end of the window.

"After a very extensive review of the last almost 48 hours now, the team came to the conclusion today that the cloud that we saw over fuel cell No. 1 is probably acceptable to fly with," said shuttle Program Manager Wayne Hale. "We had another very fun and interesting Mission Management Team meeting.

"I've got to tell you that if you ever wanted to see the difference between the old NASA and the new NASA, you should have been over there today. There was a chance for everyone to participate, all the data was laid out on the table for everyone to examine, from the top of the agency down to the most junior engineer.

"We had a vigorous debate and we had many people give their opinions and arguments," Hale said. "And at the end of the day, when the Mission Management Team was polled by (Chairman) LeRoy Cain, it was not quite unanimous but very nearly unanimous with just a few folks voicing concerns that we go ahead and attempt to fly."

Some engineers, including representatives of UTC Power, the builder of the fuel pumps, believed it made more sense to replace the suspect fuel cell, which would have delayed the flight until after an upcoming Russian Soyuz flight to the space station.

But Hale ultimately sided with LeRoy Cain, who argued in favor of launching during Wednesday's MMT meeting, and ordered engineers to press ahead for a Friday launch.

Facing the Bush administration's 2010 deadline to complete the space station and retire the shuttle, and with multiple delays and just two flights under its belt since the 2003 Columbia disaster, some outside observers believe NASA managers are under pressure to resume station construction as soon as possible.

NASA Administrator Mike Griffin approved a shuttle launch in July over the objections of NASA's chief engineer and safety officer about external tank foam insulation that was officially classified as "probable/catastrophic" in NASA's risk matrix. At the time, Griffin said he disagreed with that assessment and said he decided to proceed because of the potential impact of a lengthy delay on the downstream station assembly schedule.

In the wake of the Columbia accident, NASA was criticized for putting the launch schedule ahead of flight safety and succumbing to "go fever." But Hale today dismissed concern that schedule pressure played into today's decision, saying "I think that's a cockeyed way to look at it."

Hale said the fuel cell problem never violated NASA's safety guidelines and that Atlantis could have been launched, by the book, without any additional debate.

"What I would tell you - no offense - what I would tell you is without having a requirement, without having what in the old days would have been considered any credible reason at all, we have stood down two days to make sure that we are safe to fly. OK?" Hale told CBS News. "So we have gone the extra mile. And I've gotta tell you, from where I sit, I don't feel like we're racing to the end of the window. OK?

"We have a couple of days available for launch and if we get the opportunity to go fly, I would like to take advantage of those couple of days. ... I feel very confident we're not racing, that we have, in fact, taken a very slow, methodical, thorough and safety-oriented approach in this whole matter, far and above what would have been done by folks in the old days."

The goal of the 116th shuttle mission is to install a 35,000-pound $372 million solar array on the end of a massive truss stretching across the U.S. Destiny laboratory module. It is the first of a half-dozen extremely complex flights to build out the solar arrays and prepare the station for arrival of European and Japanese research modules.

Engineers initially hoped to launch Atlantis Aug. 27, but the flight was grounded by a launch pad lightning strike and then by tropical storm Ernesto. A launch try Wednesday was called off after an apparent short circuit showed up in fuel cell No. 1.

Atlantis is equipped with three fuel cells that generate direct current electricity by combining oxygen and hydrogen in a sort of reverse electrolysis. That direct current is then converted into three-phase alternating current and distributed throughout the spacecraft across three electrical buses.

The short in question occurred in fuel cell No. 1's Freon coolant pump motor. While the motor runs fine on two-phase power, a subsequent short would knock it out of action and, without cooling, force the crew to shut down fuel cell 1. A fuel cell shutdown, in turn, would trigger a flight rule requiring the crew to shorten its mission and to land early.

If a minimum-duration flight was declared early, the astronauts would still be able to dock with the station, attach the new solar array and stage at least one spacewalk to plug it into the station's power and cooling systems. After that, the crew would be ordered home early. But Hale said he believed the odds of that, while not zero, were acceptably low.

Extensive testing over the past two days convinced most engineers the short is in the motor itself and poses little risk of additional trouble. Playing it safe, a circuit breaker will be pulled for the duration of the mission that will cut phase A power to the Freon pump motor, a hydrogen pump in fuel cell No. 1 and a temperature sensor.

Fuel cells do not lend themselves to repairs at the launch pad and in any case, NASA could not access the powerplants before the end of the launch window. The only realistic options were to fly as is or to order a swap out, a procedure that would take at least 11 days to complete at the launch pad.

Hale said the complexity of the repair job raised the threat of collateral damage and added that even if Atlantis was delayed past Saturday, he would not order a swap out.

In any case, the Russian space agency plans to launch a Soyuz rocket carrying the space station's next full-time crew on Sept. 18. Any launch past the 18th would result in a pre-dawn landing for the outgoing station crew Sept. 28, a scenario the Russians are adamant about avoiding.

In earlier discussions, NASA promised the Russians Atlantis would leave the station by Sept. 17 at the latest. NASA managers and their Russian counterparts have now agreed that Atlantis can remain docked to the station through Sept. 18 if necessary. That would permit the Atlantis astronauts to launch on Saturday, if required, and still extend the flight one day for additional heat shield inspections if required.

Additional coverage for subscribers:
VIDEO: BRIEFING ON FUEL CELL DELAY DIAL-UP | BROADBAND
VIDEO: TUESDAY'S STATUS UPDATE BRIEFING DIAL-UP | BROADBAND
VIDEO: PRE-LAUNCH NEWS BRIEFING DIAL-UP | BROADBAND
VIDEO: MONDAY'S COUNTDOWN STATUS DIAL-UP | BROADBAND

VIDEO: BIOGRAPHY MOVIE ON THE SIX ASTRONAUTS PLAY
VIDEO: SHORT MOVIE PREVIEW OF ATLANTIS' MISSION PLAY
VIDEO: OUTLOOK ON UPCOMING STATION ASSEMBLY FLIGHTS PLAY
VIDEO: CREW ARRIVES IN T-38 TRAINING JETS PLAY
VIDEO: COMMENTS FROM CREW AFTER ARRIVAL PLAY
VIDEO: NEWS BRIEFING ON RETURN TO PAD DIALUP | BROADBAND
VIDEO: ATLANTIS ROLLBACK BEGINS PART 1 | PART 2
VIDEO: OFFICIALS EXPLAIN LIGHTNING SCRUB PLAY
VIDEO: SEE THE LIGHTNING STRIKE AT PAD B PLAY
VIDEO: ANOTHER VIEW OF LIGHTNING STRIKE PLAY


VIDEO: COMPLETE PREVIEW OF ATLANTIS MISSION PLAY
VIDEO: DETAILS OF THE THREE SPACEWALKS PLAY
VIDEO: MEET THE SIX ASTRONAUTS PLAY
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